1. Field of the Invention
The present invention is directed to a marker (or tag or label) suitable for use in an electronic article surveillance system of the type making use of magnetoelastic (magnetostrictive) properties of the marker in order to trigger an alarm under appropriate circumstances.
2. Description of the Prior Art
Magnetoelastic anti-theft security labels are utilized on the basis of an amorphous strip as resonator and of a magnetically semi-hard strip arranged parallel to each other for setting the resonant frequency. These magneto-elastic strips usually contain monitoring strips that are composed of a preferably amorphous alloy with high-magnetostriction and that are mechanically freely movable within a label housing.
These amorphous alloys are based, for example, on alloy systems that, in addition to containing Fe, also contain Ni (WO 90/03652) or that, in addition to containing Fe, also contain Ni and Co (Utility Model G94 12 456.6), and that are excited by an external excitation with an alternating field pulse, being excited to execute longitudinal, mechanical resonant oscillations.
Such magneto-elastically excitable labels for monitoring and identification systems that are composed of a magnetically semi-hard strip for the deactivation and of an amorphous strip as resonator are also disclosed by WO-A-96/32731.
As a result of the magnetostrictive coupling, the mechanical stresses linked to the oscillation cause a change in magnetization that induces a corresponding alternating voltage in the reception coil, the label being thus detected. The characteristic of the label is the resonant frequency of the amorphous strip that, in addition to being dependent on the length of the label, is also dependent on the constant field pre-magnetization of the magnetically semi-hard strip arranged parallel thereto. The matching of the amorphous strip in terms of its length to the flux of the magnetized, magnetically semi-hard strip determines the resonant frequency of the label in the activated condition. This resonant frequency must clearly differ from the resonant frequency of the amorphous strip without constant field pre-magnetization or with low constant field pre-magnetization, since this shift of the resonant frequency is required for the de-activation of the labels. The setting of an adequately large shift of the resonant frequency for deactivation, however, results in a not inconsiderable sensitivity of the resonant frequency to be detected to variations in the properties of the amorphous strip as well as of the magnetically semi-hard strip with respect to the pre-magnetization (for example, G. Herzer, U.S. Pat. No. 5,728,237).
According to the prior art, amorphous strips and magnetically semi-hard strips are separately manufactured and a matching of the properties is separately undertaken for both strips. Accordingly, the criteria made of the utilized strips in terms of their properties and, in particular, in terms of the deviations of the properties within large run lengths are extremely critical and lead to unsatisfactory yield values in the overall process.
According to the prior art, the labels are presented with freely oscillating, amorphous strips, so that the label length corresponds to exactly half the wavelength of the resonant frequency. In order to obtain more economical labels, the width of the strips can be reduced only within a limited scope given the resonant frequency established by the system.